This invention relates to a method for forming an end portion of a tube used for a muffler of a vehicle, etc. and more particularly to a method for forming an end portion of a flattened tube which has a short axis direction and a long axis direction in section, the flattened tube having such a configuration in section as elliptic, rectangular, or the like.
In recent vehicles, a tube having an elliptic configuration in section is often used as a tube constituting a muffler. The reason is that the tube having an elliptic configuration in section has a long axis direction and a short axis direction and therefore, it can exhibit such an advantage in respect of layout as being able to reduce the width in the up and down direction compared with a sectional area thereof by placing the tube such that the short axis direction is oriented to the up and down direction.
Incidentally, an exhaust tube connected to the muffler has a circular configuration in section and in addition, it has a smaller diameter than the elliptic tube constituting the muffler. For this reason, as shown in FIGS. 16 through 18, a taper-like tip reduced portion a and a connecting portion b having a circular configuration in section and having an even diameter are formed on opposite end portions of an elliptic tube A constituting the muffler by spinning. The connecting portion b is fittingly connected to an exhaust tube (not shown).
When the tip reduced portion a is formed on the end portion of the elliptic tube A by spinning, a projection c projecting in a short axis direction is, as shown in FIG. 17, formed on an intersection part between the outer periphery of the elliptic tube A in the short axis direction and the outer periphery of the tip reduced portion a on the enlarged diameter side end portion. This projection c brings about such a problem that it interferes with other members of the vehicle. In order to avoid this problem, it is generally practiced that a post machining is executed in order to depress the projection c so as to be roughly flattened after the process for machining the tip reduced portion a. However, in the case where a projecting amount d1 of the projection c is overly large, the area of the intersection part where the projection c is formed is depressed or a waving phenomenon occurs in which wrinkles are formed on the tip reduced portion a, after the post machining. Therefore, it is demanded that the projecting amount d1 is diminished as much as possible.
The present invention has been made in order meet with the above-mentioned demand. According to the present invention, there is provided in a method for forming, on an end portion of a tube stock, a tip reduced portion having a circular configuration in section which is gradually reduced in diameter from a center side of the tube stock towards an end edge side thereof by relatively rotating a forming tool about an axis of the tube stock and moving the forming roll towards the axis of the tube stock as the forming roll is moved from the center side of the tube stock towards the end edge side thereof while pressing the forming tool against an outer periphery of the end portion of the tube stock which outer periphery has a sectional configuration having different lengths in two mutually orthogonal directions, a method for forming a tube end comprising the step for forming the tip reduced portion of the tube stock such that an angle of inclination of an outer periphery of the tip reduced portion with respect to an axis of the tube stock is gradually reduced from the center side of the tube stock towards the end edge side thereof so that a generating line of the outer periphery of the tip reduced portion becomes a center recessed curve.
It is accepted that the forming process by the forming tool is repeated plural times so that the tip reduced portion is gradually finished from the center side of the tube stock in every forming process or that through repetition of the forming process plural times using the forming tool, the end portion of the tube stock is reduced in diameter in every forming process and the entirety of the tip reduced portion is formed in a final forming process.